Topic 1 - short answer responses Flashcards
Describe the structure of DNA
DNA is a double helix/double stranded molecule of DNA nucleotides. DNA forms a ladder-like structure with the sides corresponding to the sugar phosphate backbone and rungs to the bases. the bases are complementary with adenine pairing with thymine and guanine pairing with cytosine
How are DNA strands held together? And why is this important?
By weak hydrogen bonds which is important for DNA processes of replication and transcription so that the enzymes can seperate easier to expose the bases to pair with free nucleotides.
Describe how DNA is packaged in the nucleus
Histone proteins bind to the DNA coming together to form nucleosomes which then become tightly coiled forming chromatin which allows the DNA to become compacted in the nucleus.
Describe DNA replication in Eukaryotic cells
DNA replication happens when DNA helicase unzips the DNA by breaking the weak hydrogen bonds. Both strands act as templates. Free DNA nucleotides bind to the exposed bases by complementary base pairing via hydrogen bonding. Nucleotides joined by DNA polymerase resulting in 2 DNA molecules which are genetically identical each composed of one old strand of DNA and one newly synthesised strand.
“DNA replication is semi-conservative” describe the meaning of this phrase
DNA replication is referred to as semi-conservative because both strands are used as templates resulting in two genetically identical DNA molecules composed of one old and one newly synthesised strand.
Explain the importance of complementary base pairing
Since A only pairs with T and C only pairs with G, this ensures that when the two strands are used as templates, the two daughter cells will be genetically identical.
Explain the relationship between the coding and template strands of the DNA
The gene or coding strand is complementary to the template strand and it is the template strand that is used by RNA polymerase to make a complementary RNA.
Describe the process of transcription
RNA polymerase binds to the promotor in front of the gene separating the two strands to expose the bases on the template strand so that free RNA nucleotides can bind by complementary base pairing. RNA polymerase joins the nucleotides and this continues until it reaches a terminator. The mRNA molecule is released ending transcription.
Describe mRNA’s role in protein synthesis
Carries the information encoded within a gene (DNA) to the ribosome to direct the synthesis of a specific protein.
Describe ribosomes role in protein synthesis
Read the mRNA to direct the binding of specific tRNA molecules by codon - anticodon pairing to deliver specific amino acids. The ribosomes join these with polypeptide bonds.
Describe the role of tRNA in protein synthesis
Each tRNA brings a specific amino acid to the ribosome dependent upon its anticodon pairing with a corresponding codon on the mRNA
Describe the relationship between DNA codons, RNA codons, Anticodons, and amino acids
A DNA codon is a sequence of 3 nucleotides within the gene (coding strand) which codes for an amino acid or stop codon. An RNA codon is a set of 3 nucleotides on mRNA coding for an amino acid or stop codon (complementary to the template strand). Anticodons are found on tRNA and recognise a codon on mRNA through complementary base pairing (so they can deliver specific amino acids to the ribosome). Amino acids are the subunits of polypeptides joined together by peptide bonds.
Describe how the 3D shape of a protein is determined
The primary structure is a unique set of amino acids joined by peptide bonds. Small regions of amino acids can form the secondary structure (a helix and b sheet) by interactions between the side chains of amino acids. The tertiary structure is the final folded form of the polypeptide formed by interactions between the regions of secondary structure and the side chains of the amino acids. If more than 1 polypeptide interacts then they form a quaternary structure. This has a specific shape.
Explain why DNA replication is given that name
This is a semiconservative DNA replication. It is given that name because both strands are used as templates to produce two genetically identical DNA molecules each composed of one old/pre existing strand and one newly synthesised strand.
Explain how enzymes lower the activation energy of chemical reactions
Enzymes lower the activation energy of a reaction in 2 ways. 1) in a catabolic/breakdown reaction when the substrate binds to the active site, the enzyme undergoes a conformational change, stressing the bonds in the substrate making it easier for the bonds to be broken and for the reaction to occur and thus lowering the activation energy. 2) In an anabolic/synthesis reaction, substrates can only align in the correct orientation in the active site for product to form, so that when the enzyme undergoes a conformational change, it can form the bonds more readily and thus lower the activation energy.
Explain how temperature affects the rate of enzyme activity
As the temperature of the enzymatic reaction increases so does the kinetic energy of the molecules (substrate and enzyme) leading to more productive collisions. at the optimum temperature continues to increase, the rate of collision becomes too fast, and the vibrational energy within the enzyme increases breaking bonds leading to a change in the shape of the enzyme such that the active site no longer recognises the substrate. resulting in a loss of activity. if the temperature continues to rise, the enzyme will unfold and become denatured permanently losing activity.
Explain how pH affects the rate of enzyme activity
At optimal pH the changes on the substrate and the active site of the enzyme are opposite leading to attraction and enzyme activity, product formation. However, as the pH moves away from the optimum (too high or too low) their charges become more similar resulting in repulsion and a loss of enzyme activity. at extremes of pH, the charge interactions in the enzyme are also affected leading to unfolding of the protein and denaturation and loss of activity.
Explain how chemical inhibitors affect the rate of enzyme activity
There are two types of chemical inhibitors. competitive inhibitors have a similar shape to the substrate and are thus complementary to the active site and so are able to bind and block the substrate from binding thus preventing a reaction. Non-competitive inhibitors so not bind to the active site but bind ar an alternate site (allosteric site) resulting in a change in the shape of the enzyme including the active site so that it is no longer able to bind the substrate, and hence no reaction occurs.
What is feedback or end product inhibition?
Many metabolic pathways are controlled/regulated by feedback inhibition. this is when the product of a pathway can inhibit its own synthesis. if there is excess product, it can bind to one of the early enzymes on the pathway, changing its shape so that the substrate cannot bind. that is it blocks the reaction. cell does not need to waste energy making more product than it needs.
explain how hormone or growth factors only recognise particular cell types
Hormones/growth factors bind to complementary shaped binding sites on their receptors.
If a cell does not have the corresponding receptor the growth factors of hormones can not bind.
Explain how an antibody functions to identify an antigen
Antibody molecules are specifically for their corresponding antigen. their quaternary structure creates antigen-binding sites which are complementary to the shape of an antigen.
Immunisation/vaccination results in an antibody response to produce antibodies against antigens on a specific pathogen, which can then bind to the pathogen and prevent infection.
Describe the process by which cells become specialised
The process by which cells become specialised is called cellular differentiation. In specialised cells some genes are turned on or off so that they have a pattern of gene expression specific for the cell type. This pattern determines cell structure and function, eg. muscle cells express myosin which determines the shape and function of a muscle cell.
List the requirements for the polymerase chain reaction
- DNA sample to act as the template for new DNA synthesis
- 2 DNA primers complementary to the ends of the region to be amplified.
- DNA nucleotides incorporated into new DNA
- heat resistent DNA polymerase to synthesise new DNA
- Buffer for the optimum pH for DNA polymerase
- thermal cycler to do the heating/cooling/heating
describe the properties and role of the primers used in the polymerase chain reaction
the 2 DNA primers are short stretches of DNA complementary to the ends of the region to be copied. they need to be sufficiently specific so that they only occur once in the genome. they determine the size of the fragments made by defining the starts and also identify where DNA polymerase must bind to start the synthesis of new DNA.
